A MiniMegaChip/MiniMegiChip/MegaRAM will be expensive. Grab a 4Mb half-length FAST memory expansion for your A2000 and be happy.
They are pretty cheap and will make wonders for your A2000.
Better than this is grabbing/renting/robbing/buying a SCSI controller with some memory on it, like the A2091 (auto-boot if ROM is 1.3 or better, have up to 2Mb FAST RAM) or (the better) GVP HD8+ (Fast SCSI, up to 8Mb with ordinary 30-pin 1Mb SIMM).
